Cylindrical surface horn forming a transition between a closed periodic circuit and an open sided periodic circuit



Y w YR m ma 0 d F J .w P I 1 F 3,513,416 CYLINDRICAL SURFACE HORN FORMING A TRANSITION BETWEEN A CLOSED U I I' I I I I I I I I I May 19, 1970 PERIODIC 01130111": AND AN OPEN SIDED PERIODIC cmcun 9 F g: I II I FIG. 3

FIG. 2 PRIOR ART PRIOR ART slo FIG. 6 2.6

FREQUENCY (G Hz) FIG 7 ORNEY FIG. 4

United States Patent Office 3,513,416 Patented May 19, 1970 US. Cl. 333-73 3 Claims ABSTRACT OF THE DISCLOSURE A high power harmonic microwave filter is provided with openly flared horn transition sections. The flared portions of the horns are curved surfaces conforming to sections of a cylinder with a radius of curvature greater than 5 free space wavelengths at the lower cut-off frequency of the passband of the filter. The horn sections match the ends of a main filter section of open sided ladder line slow wave circuit to intermediate transition sections of ladder line enclosed by a waveguide. The curved surface of the horn is tangent to the wall of the enclosing waveguide section at the junction therewith. The horns have a flared opening with a height of at least 1 free space wavelength above the surface of the open sided ladder line. The enclosed ladder line transition sections are in turn matched to waveguides which feed microwave energy to and from the filter. Energy absorbing elements are positioned over the main filter section inbetween the horn transitions for absorbing unwanted harmonic power outside the passband of the main filter section, which unwanted power tends to be radiated from the main filter section and from the input horn section. The main filter section is curved to prevent line of sight transmission from the input horn transition to the output horn transition.

DESCRIPTION OF THE PRIOR ART Heretofore, open sided periodic slow wave circuits have been employed in microwave filters. Such filters comprise a waveguide which is matched to an open sided slow wave circuit via the intermediary of a first transition section which incorporates the slow wave circuit into a length of the waveguide, thereby forming a closed section of periodic slow wave circuit. The closed section of slow wave circuit is outwardly flared to form a horn section which opens toward the open sided slow wave circuit portion. A similar horn, closed periodic slow wave section, and section of waveguide, in that order, are located at the other end of the open sided slow wave circuit. The open sided section of slow wave circuit is enclosed at a substantial distance by a wave energy absorbing material. In operation, wave energy within the passband of the filter travels from one end of the filter to the other without substantial reflection or absorption. However, harmonics of the passband energy and other energy outside the passband is radiated by the open sided section of periodic slow wave circuit and is absorbed by the surrounding lossy material. This radiated energy is not transmitted between the horn structures because the horns are displaced relative to each other to prevent line of sight transmission therebetween. 40 to 60 db attenuation of the harmonics is obtained relative to the energy within the passband of the filter.

In these prior art filters, the horn transitions have had a linearly tapered outward flare. The prior art linear taper of the horns has two defects, reflection of energy because such a taper is not a good transformer, and scattering of energy from the discontinuous junction. The function of the horn is to transform the closed slow wave structure to an open slow wave structure by removing the enclosing boundary to infinity. In the process of going from a narrowly enclosed propagating mode, whose propagation characteritsics depend greatly on the dimensions of the boundary (i.e., a waveguide mode), to an open propagating mode which depends only on the reactive surface, a basic mode transformation is required. This must be done very gradually because if the wave energy encounters a discontinuity in the enclosing boundary while it is still to some extent dependent on this boundary, higher order modes of wave energy will be set up which satisfy the boundary conditions at the discontinuity. Since the transverse dimensions of the horn are increasing with distance along the propagating axis, these modes may not be evanescent (as at a discontinuity in uniform dominant mode waveguide) but rather, capable of propagation out of the horn. Therefore, the essence of a useful horn is that it be gradual enough to prevent the forward propagation of higher order modes, i.e., scattering. Obviously a discontinuity is not gradual, and a linear horn necessarily involves a discontinuity at the junction. It has been found that this linear taper of the horns causes a substantial reflection and scattering of the wave energy within the passband of the filter. It is desirable to eliminate this scattering and reflection as they produce substantial attenuation of the energy within the passband of the filter.

SUMMARY OF THE INVENTION In the present invention it has been found that, in the aforecited filter circuits, the unwanted reflection and scattering of wave energy, within the passband, from the horn structures is eliminated by forming the opening flared portions of the horns with curved surfaces formed by cylindrical surfaces tangent to the walls of the enclosing waveguide sections to which the horn is joined. The cylindrical horn surfaces have a sufliciently large radius of curvature to prevent excessive reflection or scattering of wave energy within the passband of the filter. A suitable radius of curvature is one in excess of 5 free space wavelengths at the lowest frequency within the passband of the filter.

The principal objects of the present invention is the provision of an improved horn transition section for opening into an open sided periodic slow wave circuit from an enclosed slow wave circuit.

One feature of the present invention is the provision of a horn having openly flared cylindrical surface sections forming the transition structure between a closed section of periodic slow wave circuit, propagating wave energy in a unique mode, and an open sided section of periodic slow wave circuit, whereby wave energy within the passband of the open sided slow wave circuit passes through the horn transition without substantial reflection or scattering.

Another feature of the present invention is the same as the preceding feature wherein the open sided slow wave circuit is a ladder line.

Another feature of the present invention is the same as any one or more of the preceding features wherein the open sided slow Wave circuit is a thick ladder line closed on one side and open on the other.

Another feature of the present invention is the same as any one or more of the preceding features wherein similar horn transition sections are provided at opposite ends of the section of the open sided periodic slow wave circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the cylindrical horn surfaces have a radius of curvature in excess of 5 free space wavelengths at the lowest frequency within the passband of the open sided slow wave clrcult.

Another feature of the present invention is the same as the preceding features wherein the horn has a height above the slow wave circuit at its enlarged open end in excess of 1 free space wavelength at the lowest frequency within the passband of the slow wave circuit.

Other features and advantages of the present invention will become apparent upon a perusal of the following specifications taken in conjunction with the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional line diagram of a prior art filter,

FIG. 2 is a schematic plan line diagram of the structure of FIG. 1 taken along line- 22 in the direction of the arrows,

FIG. 3 is an enlarged sectional view of alternative portion of the structure of FIG. 1 delineated by line 3-3 and depicting the horn structure of the present invention,

FIG. 4 is a sectional view of the structure of FIG. 3 taken along line 44 in the direction of the arrows,

FIG. 5 is an 02-5 diagram for the structure of FIG. 3,

FIG. 6 is a plot of loss in db versus frequency for the prior art structure and for the structure of the present invention, and

FIG. 7 is a schematic line diagram of a filter of the present invention.

Referring now to FIGS. 1 and 2, there is shown a prior art microwave filter 1. The filter includes an input section of rectangular waveguide 2 which tapers into and is impedance matched to a closed section of rectangular waveguide 3 enclosing a periodic slow wave circuit 4 such as, for example, a thick ladder line circuit.

A horn transition section 5 of closed waveguide surrounds the slow wave circuit 4 and is joined to the end of the closed waveguide section 3. The horn 5 has a linear taper, is outwardly flared, and opens into a section 6 of the slow wave circuit which is open sided in that no conductor surrounds the slow wave circuit in sufliciently close proximity to appreciably influence the propagation characteristics of the slow wave circuit 4 for the dominant mode of propagation of wave energy within its passband.

Similar horn, closed slow wave circuit and waveguide sections 5', 3 and 2' are provided at the other end of the filter 1. Non-reflective wave energy absorbing elements 7 and 8 are disposed over the open sided slow wave section 6 in between the horn structures 5 and 5'.

In operation, wave energy to be filtered is applied to the input end 9 of the filter 1. The wave energy is coupled onto the slow wave circuit 4 within the closed section. of guide 3. The Wave energy within the passband of the slow wave circuit propagates along the slow wave circuit 4 through the transition horn section 5 and thence along the open sided slow wave circuit section 6 to the output horn transition 5, closed waveguide section 3', and out the waveguide 2'. l

The wave energy within the passband of the slow wave circuit 4 sticks on the circuit 4 and is not appreciably attenuated in its travel through the filter 1. However, Wave energy outside the passband of the periodic slow Wave circuit 4 is radiated from the open sided circuit beginning at the first horn transition 5 and such radiation continues over the length of the open sided circuit section 6. This radiated energy is absorbed by the absorbing elements 7 and 8. As a consequence, the unwanted harmonic content and other spurious signal content of the input signal is absorbed by the lter 1 while passing the desired signals through the filter without substantial attenuation. These unwanted signals are typically attenuated by 40 to 60 db relative to the attenuation of the signal energy within the passband of the filter 1. A typical plot of loss versus frequency over the passband of the filter 1 is shown in FIG. 6.

The prior art curve shows an attenuation of 0.4 db over a passband of about 11%. For many high power filter applications, it is desirable to reduce the attenuation over a wider passband to even lower amounts such as, for example, to less than 0.3 db over a band of 15%.

Referring now to FIGS. 3 and 4, it has been found that less of the desired wave energy within the passband will be radiated from the slow wave circuit 4 the horn transition section 5 has a cylindrically flared surface instead of a linear taper. In particular, the broad and narrow waveguide walls 15 and 16 of the horn section 5 should start out tangentially to the enclosing waveguide walls 17 and 18. It has been found that a suitable taper is formed when the walls 15 and 16 are formed by sections of cylinders having a radius of curvature R of 24" and which tangentially mate to the closed waveguide section 3 propagating in a unique slow wave mode. The cylindrical walls 15 and 16 start out tangentially to the enclosing waveguide walls 17 and 18. The tangential junctions between the walls 17 and 18 of the waveguide and the cylindrical walls of the horn 15 and 15 prevent wave reflection and scattering at the junction. In addition, the relatively large radius of curvature R of at least 5 free space wavelengths at the lowest passband frequency of the slow wave circuit, prevents setting up propagating wave modes within the horn. The horn section should have an axial length sufiiciently great such that the open end of the horn has a height h of at least 1 free space wavelength at the lowest frequency wi hin the passband of the slow wave circuit. When the horn has cylindrical wall surfaces and dimensions as aforecited it provides a transformation from the closed slow wave circuit to the open sided slow wave circuit and minimizes scattering of passband wave energy.

In a typical example, the horn section 5 has an axial length of 13" to form a horn having maximum dimensions at its widest point of 11" x 5.8". Such horn structures 5 and 5' reduce the loss of wave energy within the passband of 2.6 gHz. to 2.95 gHz. to less than 0.3 db, as shown by curve 19 of FIG. 6, and provide a substantial improvement in the performance of the filter 1.

The slow wave circuit 4 may take any one of a number of forms which will provide an open sided structure when the enclosing waveguide structure is removed. One class of suitable periodic slow wave circuits includes the ladder line circuits wherein a succession of slots are cut through or provided in a conducting sheet to form an array of periodic Coupled elements.

A preferred ladder line for high power levels, i.e., 1.6 mw. peak and 0.00072 duty factor, is a thick ladder line circuit as depicted in FIGS. 3 and 4.

In the thick ladder circuit 4, the slots 26 are provided in a thick conductive sheet 27 as of copper with the slots 26 open at one side and closed at the other by not passing through the sheet 27. The pitch p, i.e., distance from one slot to the next, determines the desired upper stop band frequency, 0. The slot length 2b determines the low frequency cutoff (.0 see FIG. 5. The slot depth 1 determines the upper cutoif frequency w which occurs when l is approximately one quarter guide wavelength. The slot width w determines the amount of stored energy in the slots 26.and should be made as large as practical. The width of the ground plane or sheet 27 has little eifect provided it isgreater than 3b.

The slot length 2b is preferably restricted sufliciently to prevent the existence in the slot 26 of the TE mode in the passband. The maximum value for 5 is, thus, p/ 2b and in turn fixes the lower cutoff frequency (.0 of the ladder line. The pitchp is set at a value corresponding to one half a wavelength at a slightly lower frequency than the second harmonic frequency of the lowest frequency of the passband of the filter. For a filter operating from 2700 mHz. to 2900 mHz. the second harmonic frequency is 5400 mHz. and the pitch is selected corresponding to a half wavelength at 5370 mHz. which is 1.1". The slot length 2b is made 3.7" corresponding to a lower cutoff frequency of 1595 mHz. and places the TE cutoff at approximately 3190 mHz. The slot depth l is set at 0.74. The webs 28 disposed between adjacent slots are made 0.250" thick and provided with a full radius at their ends.

The closed section of tapered waveguide 2 and 2 has an input terminal 9 for mating with WR284 guide and has a terminal height, at the junction with section 3, of 1.60" and a terminal width of 3.70". The waveguide 2 and 2' has a dispersion characteristic 20, see FIG. 5. The closed section of slow wave circuit 3 has a height of 1.4" above the web tips and is at least 2 and preferably 2.5 periods long with a dispersion characteristic 21. The horn section 5 has a length of 13 inches which is 8.5 periods long. The open sided section of slow wave Circuit 4 has a length of 74 periods with a dispersion characteristic 22. The sheet 27 is 20.1" wide. The open sided slow wave circuit 4 is curved, as shown in FIG. 7, and includes a straight section 31 of 31 periods and a curved section 32 of 43 periods. The curved section has a radius of curvature of 108". The curved section 32 places horn 5' out of a line-of-sight transmission path across the circuit 4 between the horns 5 and 5 to prevent picking up energy radiated from horn 5. An energy absorbing lossy structure 33 is disposed over the open sided circuit 4 between the horns 5 and 5 for absorbing energy outside the passband of the circuit 4 which energy is radiated from the circuit 4 and from the horn 5.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave apparatus including, means forming a closed section of slow wave circuit defined by a conductive rectangular waveguide structure enclosing a periodic slow wave structure, said rectangular wave guide having a broad top wall and a pair of narrow side walls, means forming an open sided section of slow wave circuit defined by a section of periodic slow wave structure and a horn-shaped waveguide section of rectangular cross section having a broad top wall and a pair of narrow side walls all enclosing said section of slow wave structure, the improvement wherein, said horn section of rectangular cross section waveguide of said horn transition section is formed by cylindrical wall sections with the axes of revolution of said cylindrical wall sections being disposed orthogonal to the axis of said waveguide and which sections start out tangent with said broad top wall and both of said narrow side walls of said enclosing waveguide structure, said horn being openly flared in a direction taken from said closed section of slow wave circuit and toward said open sided section of slow wave circuit to provide a substantially refiectionless and lossless transition thereto within the passband of said open sided slow wave section, a second horn transition section of closed periodic slow wave circuit being connected to the opposite end of said open sided section of periodic slow wave circuit from said first horn transition section for completing a path of microwave propagation through said open sided section of periodic slow wave circuit, means forming a wave energy absorbing structure disposed intermediate said first and second horn transition sections and spaced from said open sided slow wave circuit such that it absorbs Wave energy radiated from said open sided section of periodic slow wave circuit without substantially absorbing wave energy within the passband of said open sided slow wave circuit, said section of open sided slow wave circuit being curved to enhance radiation of wave energy outside the passband of said open sided slow wave circuit and to prevent formation of a line of sight transmission path for radiated energy between said first and second horns.

2. The apparatus of claim 1 wherein said cylindrical side wall portions of said horn structure have a radius of curvature in excess of 5 free space wavelengths at the lowest frequency within the passband of said open sided slow wave circuit.

3. The apparatus of claim 1 wherein said horn structure has an axial length suificient to provide a height above the open side of said open sided slow wave circuit of the upper wall of said horn at its open end which is in excess of 1 free space wavelength at the lowest frequency within the passband of said open sided slow wave circuit.

References Cited UNITED STATES PATENTS 2,736,866 2/ 1956 Clavier et al. 2,737,632 3/1956 Grieg. 2,869,085 1/ 1959 Pritchard et al.

OTHER REFERENCES Zucker, The Guiding and Radiation. of Surface Waves, Symposium of Modern Advances In Microwave Techniques, 1954; pp. 403 and 416.

HERMAN K. SAALBACH, Primary Examiner T. VEZEAN, Assistant Examiner US. Cl. X.R. 

